The long term objective of this proposal is to increase our understanding of immune responses to polysaccharide antigens. To accomplish this goal, we will measure serum antibody responses to bacterial PS vaccines, and in vitro Ig production by mitogen stimulated lymphocytes from normal and abnormal subjects. In Aim 1, we will investigate the immunologic and genetic basis of "vaccine failure" in patients who develop Haemophilus influenzae type b (Hib) disease despite previous vaccination with Hib polysaccharide (PS) vaccine. This group previously has been shown to have impaired serum antibody responses to Hib PS despite having normal concentrations of serum Ig. The techniques we propose to use to evaluate these patients include: a. measurement of their IgM, IgG and IgG subclass-specific antibody responses to immunization with different bacterial PS antigens presented as isolated PS vaccine (T-independent) or as a covalent PS-protein conjugate (T-dependent); b. measurement of their IgG antibody responses to Hib outer membrane proteins following recovery from Hib disease; c. measurement of Ig secretion in vitro by mitogen- stimulated lymphocytes; and d. determination of the frequencies of different allotype haplotypes previously associated with altered risk of Hib PS vaccine failure. The results of these studies will define the range of the immunologic deficits present in these children, and the extent to which genes in linkage disequilibrium with Ig allotypes contribute to vaccine failure. In Aim 2, we will immunize healthy adults with meningococcal A and C PS and tetanus toxoid vaccines and compare their respective IgG2 antibody responses in relation to G2m(23) allotype status to determine whether the relationship observed previously in normal individuals between this allotype and the magnitude of the IgG2 antibody responses to Hib PS is specific or extends to IgG2 responses to other PS antigens or to protein antigens. We also will compare the amount of IgG2 produced in vitro by mitogen-stimulated lymphocytes from G2m(23)-positive and negative subjects in an effort to develop an in vitro assay to investigate T and B cell interactions contributing to this allotype association. Finally, in Aim 3, we will compare the relative bactericidal and opsonic activities of IgG1, IgG2 and IgG3 antibodies to Hib PS. Our previous data suggest that genetic factor as well as age of immunization and type of vaccine influence the IgG subclass distribution of serum antibody to Hib PS. The results of these studies will indicate whether individuals with similar levels of total IgG antibody but different subclass distributions are equally protected against Hib disease.